Great question! I too have been wondering this myself. 

One solution I have found is, going to the date/time on the text messsge they have requested on your appointment app and when clicking on clients name, instead of hitting accept or decline, you can message them and tell them that the time needs to be adjusted before accepting.  If I get back to a client relative to the time they booked appointment they get back to me in a timely matter reguarding recently booked appointment! I hope this helps!